This invention relates to a method and apparatus for regulating the armature currents of a plurality of motors driving the same load so that the motors are caused to operate at substantially the same speeds and load division among the motors is substantially equalized.
There are a variety of applications wherein a plurality of motors are required to drive a common mechanical load; examples of such applications occur in excavating machines wherein a variety of motions such as swing, hoist, drag or crowd might be so driven. However, the application of this invention is not restricted to excavating machinery.
It is theoretically possible that initially any load can be driven by a single motor assuming that motor could be constructed to have a sufficient horse power value to operate the load. For very large loads such motors would be excessively large, and, therefore, have a large inertia mass. In order to overcome such inertia mass problems and enable faster acceleration and deceleration it is common to use several smaller motors, instead of the single motor, the totality of the smaller motors possessing the requisite power for driving the load in question. The use of a plurality of motors in this fashion produces other advantages including reduction of gear stresses and providing for the possibility of continuation of operation should a single motor fail.
When a plurality of motors are connected in common to a given load, it is necessary that the motors have the same rotational speeds. Prior art demonstrates a number of ways for regulating the speeds of such motors relative to each other to produce the requisite sameness, but these prior art techniques have serious disadvantages.
The traditional technique for providing control or regulation of load sharing motors of the direct current variety which, as stated, must run at the same speed to ensure proper load sharing, is through the use of a so-called "sandwich series" circuit. In such a circuit the motors and generators provided for producing the armature currents for the motors are connected alternately in series in a loop. There is a source of excitation for each motor field and a separate source of equal excitation for each generator field. This arrangement is commonly used because it produces excellent load division between the individual motors. However, a serious disadvantage results in this system because it permits torsional oscillation at resonant frequencies which lie in the operating range of the mechanism in question. Clearly, these oscillations are undesirable since they increase the loads on gearing.
The conventional "sandwich series" circuit is relatively incapable of damping oscillations between the motors. Such oscillations can reach such a severity that a phase angle between them of 180.degree. is produced, i.e., in a given two motor system the first motor is at the maximum speed excursion above a mean value and at the same time the second motor is a minimum speed below the latter mean value. The reason for this state of affairs is that the counter electromotive forces (cemf) produced by the motors are additive, and the excursions in the cemf are complimentary. The excursions exactly cancel each other, when the aforementioned 180.degree. phase displacement occurs, and the total voltage around the loop circuit is unaffected by the oscillations. Accordingly, the loop currents cannot change in a direction to permit the development of damping torques.
In order to improve the aforementioned situation it is known that if the motors in a given system are powered separately, i.e., are parts of separate individual Ward-Leonard type loops, the generators will be able to provide damping for the torsional oscillations of the drive system. In such an arrangement the motor speed variations would cause variations in the cemf which, when compared to a relatively stable generator voltage, causes substantial changes in armature current which damp the oscillations.
The use of individual loops as mentioned immediately above, however, have been thought to be less satisfactory from a load division viewpoint. It is known in the prior art that the common load drive system of the type wherein each motor has its own individual power source needs to be regulated in some fashion to insure proper load division. An attempt at a solution of this problem is discussed, for example, in U.S. Pat. No. 3,688,167. The latter patent describes a system wherein each generator field is separately excited, and this excitation is separately regulated for adjusting the speed of each corresponding motor. The armature currents for each of the motors are compared with a master value which is derived from the armature current of a master motor. Those motor generator sets which have armature currents which deviate significantly from the master value are regulated in the usual manner by adjusting the generator field voltage to correspondingly adjust the speed of associated motor. This system has significant disadvantages in that significant errors in the division can occur which will not be corrected for periods of time which are far too long. Furthermore, this arrangement does not significantly improve the torsional oscillation problem discussed above.
It is, therefore, an object of this invention to provide a method and apparatus for regulating the armature currents of electric motors which are required to drive a common load so that they will operate at substantially the same speeds providing optimal load division among the motors while substantially removing torsional oscillation in the system.